Delay lines for crossed field tubes



June 2, 1959 D. REVERDIN DELAY LINES FOR c ossED FIELD TUBES 3Sheets-Sheet 1 Filed May 5, 1955 June 2, 1959 D. REVERDIN DELAY LINESFOR CROSSED FIELD TUBES Filed May 5, 1955 5 Sheets-Sheet 2 J n 2, 1 D.REVERDIN' I 2,889,488

DELAY LINES FOR CROSSEID FIELD TUBES Filed May 5, 1955 3 Sheets-Sheet 3United States Patent DELAY LINES FOR 'CROSSED FIELD TUBES DanielReverdin, Paris, France, assignor to Compagnie Generale de TelegraphicSans Fil, -a corporation of France Application May 5, 1955, Serial No.506,331 Claims priority, application France May 13, 1954 6 Claims. (21.315-393 The present invention relates to electron discharge tubes havingcrossed electric and magnetic fields, this designation including bothtraveling wave amplifying or oscillating tubes and magnetrons whether ofrectilinear or of circular structure, and more particularly to delaylines for use therein. 7

Known tubes of this kind generally comprise an anodic electrode in theform of a delay line along which is propagated a high frequencyelectromagnetic wave in interaction with a beam. This line, rectilinearor circular is of geometrically periodic structure and compriseselements such as vanes, separating teeth between anode cavities, fingersof an interdigital delay line, rods of a ladder line, etc. e

.It is known that under the action of intense hyperfrequency fieldsprevailing in the interaction space of these tubes, and under the actionof very high densities of space charges normally used, the electron.beam, instead of being propagated in a direction parallel to the delayline, progressively approaches this delay line, so that some electronsimpinge upon the latter. As will be shown hereinafter, the impact on thelongitudinal faces of the delay line by the electrons may not constitutea major disadvantage provided that the dissipation of heat generated inthe delay line is sufficient, but theimpact on the transverse faces iscapable of creating a secondary emission which reduces the output of thetube and increases the danger of breakdown. e I The object of theinvention, which is to avoid this secondary emission, is attained byproviding a tube with crossed electric and magnetic fields whichcomprises a negative or cathodic electrode and a delay line or anodic inFigs. 3, and

electrode parallel thereto and which includes elements, j

each of which has a first transverse face parallel to the 131m definedby said fields and being the first to be impinged upon by the electronsof the beam, asecond transverse face and at least one longitudinal faceintersecting respectively the first, and the second trans- 1 verse facesalong a first and asecond. edge, sai d twoedges being parallel to themagnetic field, the distance between the. first edge and the negativeelectrode being. greater than thedistance between the second edge andthe negative electrode. p I The invention will be better understood fromthe ensuing description with reference to the accompanying drawings, inwhich: v

Fig. 1 shows in section a known form of delay line in the tubes inquestion, this figure being intended to indicate the disadvantages ofknown systems; ".Figs. 2, 3 and 4 show, respectively, in section three{delay lines embodying the invention;

Figsl S and 6 show respectively in cross-section and in axial section anoscillator magnetron tube in which the delay line shown in Fig. ,2 isincorporated;

Fig. 7 shows in longitudinal section a traveling wave Fig. 8 shows inplan View a ladder type delay line suitable for the tube shown in Fig. 7and having the form shown in Fig. 3.

Fig. 1 is a longitudinal section View of part of a delay line havingvanes 2 of known form, the vanes having, in the plane of the drawing, arectangular shape comprising transverse faces 9 and 10 and alongitudinal face 12. A face 11 separates two successive vanes. Thisdelay line or anodic electrode is brought to a positive potentialrelative to a cathodic electrode or sole 3 disposed opposite andparallel to the faces 12, so that an electrostatic field E, directedfrom the sole 3 towards the face 12 and having lines of force 21, isestablished between the delay line and electrode 3. Further, theinteraction space between the two electrodes is immersed in the magneticfield B which is perpendicular to the plane of the drawing and isdirected from the observer towards this plane. The electron beam 7 ispropagated in the interaction space and, in the absence of ultra-highfrequency field, its direction of propagation is parallel to the surfaceof sole 3 and to the faces 12 that is, perpendicular to the crossedfields E and B.

Under dynamic conditions of operation, as has been indicated above, thebeam progressively approaches the anodic electrode and some electronseventually impinge upon the latter at a small angle of incidence 0relative to the direction of the faces 11, this angle depending on theconditions particular to the tube in question and which in general doesnot exceed 20.

The ends of the trajectories of these electrons, which are assumed to beparallel and rectilinear, have been shown in Fig. 1. It can be seen thatsome electrons strike the faces 12 and others the faces El which face inthe direction generally opposite the direction of electron propagationin the interaction space between the cathodic electrode 3 and the delayline.

The fact that the electrons impinge the faces 12 does not constitute amajor disadvantage, provided that the anode is capable of dissipatingthe heat thereby generated. The secondary electrons which might resultfrom the impact of the electrons of the beam cannot leave these facesowing to the constant. electrostatic field directed towards the faces12. This field is very strong relativeto the amplitude of the ultra-highfrequency field of the wave propagated in the anode, so that theresultant electrostatic field is never nil, and is always of suificientintensity for blocking the secondary emission. On the other hand, in thespace between the vanes, the electrostatic field is nil or very weak, asevidenced by the distribution of the lines of force; thus the ultra-highfrequency field, in a part of its period, is capable of rendering theface 10 positive relative to the face 9, which creates conditions thatfavor secondary emission on the face 9 when the latter is bombarded bythe primary electrons; The above-described phenomena have been ascer'tained experimentally. They are very harmful as concerns the output ofthe tube and the output power, since the secondary electrons subtractenergy from the ultrahigh frequency field. Furthermore, they tend tocause high frequency arcs to be set up between the elements i of thedelay line and in consequence limit the high-l re;

quency voltage at which the'tube may operate without danger ofbreakdown,

" Further, the secondary electrons emitted from the face 9 followincurved trajectories 22 owing to the action of a plifier of rectilinearstructure usinga ladder-type delay e a the magnetic field B and fall onthe .face 9, 11 or 10, according to their velocity. If their velocity.is high enough, they may result in atertiary emission which reducesstill more the output and the gain. J

In the Figs. 2 to 8, like reference characters designate assesses likeelements in Fig. 1. In Fig. 2, the longitudinal face 12' is flat and isinclined at an angle relative to the direction of the electrode 3; theangle of incidence of the electrons relative to the faces 11 is as inthe delay line shown in Fig. 1. Thus it can be seen that, contrary tothat which occurs in the case of the delay line shown in Fig. l, theentire beam strikes the faces 12' whereas the faces 9 receivepractically no bombardment and hence emit practically no secondaryelectrons. The angle determines the difference in the height of thefaces 9 and 10. Although the delay line configuration varies slightly inthe several embodiments described hereinafter, the reference numerals 9,l0 and 11 are used in each instance to indicate the corresponding facesof the delay line vanes.

The correct choice of the angle o is obtained by trial and error.Generally speaking, it may be stated that if F is the sum of the surfaceareas of the slots 11 between the anode teeth and S the sum of thesurface areas of the faces 12, the ratio /6 increases with F /.S', thefirst being approximately equal to 1 when the second is equal to 1.

In Fig. 3, the shape of the face 12" is cylindrical, its concave sidebeing directed towards the interaction space. The height of the face 9is less than that of the face 10, the difference between these twoheights being ascertained by trial and error, as in the first example.

In Fig. 4, the face 12" is stepped. The face 12"" is formed by at leasttwo intersecting surfaces which may be for example intersecting planes,one being substantially perpendicular and the other substantiallyparallel to the face 9, the latter face thus being of less height thanthe face 1%. he difference between heights of the faces 9 and Ill isalso determined experimentally.

Figs. and 6 show respectively in cross-section and axial section amagnetron whose anode comprises a delay line 31 of the type shown inFig. 2 disposed in circular manner. The delay line 31 comprises vanes 32defined by recesses machined in the cylindrical delay line 31constituting the envelope of the magnetron. The negative electrode 33carries an emitting layer and operates as a cathode having indirectheating by a filament 34 supplied by a source 5 through leads 8. Asource 6 brings the envelope 31 to a positive potential relative to thefilament 34, which is connected to the cathode 33. The magnetic field Bis created by the polar elements of a magnet 19. The oscillating energyis extracted by means of the loop 20. The vanes 32 present flat inclinedfaces 42 to the impact of the electron beam as in the embodiment shownin Fig. 2. The inclined faces 42 each indicate an angle with respect toa tangent to a circle concentric with the cathode 33 at a point wherethe radial axis of the vane intersects the circle. By suitably selectingthe direction of the field B, the beam is made to describe a trajectoryin the clockwise direction, in the interaction space between vanes 32and electrode 33.

Fig. 7 is a sectional view of a traveling wave amplifying tube 1 withcrossed electric and magnetic fields. The tube comprises a cathodeheated by a filament 4 supplied by a source 5 and the envelope ispositively ener- 'gized relative the filament and cathode by a source 6.The electron beam from the cathode 15 passes between a delay line 44 andthe negative electrode 45 and is received by a collector 16. The delayline 44, shown in plan in Fig. 8, is for example a ladder delay line,the rods of which have a shape similar to that shown in Fig. 3. Anultra-high frequency wave is fed into the tube through an input 17, ispropagated along the delay line 44 and is received at the output 29after having been amplified, in the known manner, by interaction withthe electron beam which travels near the delay line 44. The magneticfield B, perpendicular to the plane of the draw ing, is produced bypolar elements not shown in this fixt The invention is not limited tothe embodiments de- 4. scribed hereinbefore, since it is applicable toany crossed electric and magnetic field tube, for example a backwardtraveling wave oscillator.

What I claim is:

1. An electron tube having a delay line and an electrode parallelthereto and defining therewith an electron and wave interaction space,means for positively biasing said line with respect to said electrode,thereby establishing in said space an electrostatic field having linesof force transverse thereto, means for establishing in said space atransverse magnetic field having lines of force crossed with said linesof force of said electrostatic field, and means comprising an electronsource for injecting an electron beam into said space, said beampropagating normally in a direction substantially perpendicular to saidelectrostatic and magnetic fields: said delay line comprising a seriesof periodically spaced elements, each element comprising first andsecond faces substantially parallel to each other and to the lines offorce of said electrostatic and magnetic fields, said first face facingsaid electron source, and a third face intersecting respectively saidfirst and second faces along a first and a second edge, the distancebetween said first edge and said electrode being greater than thedistance between said second edge and said electrode, so that said thirdface is turned toward said electron source, whereby electrons deviatingfrom said normal direction are absorbed by the third face of saidelements.

2. A tube in accordance with claim 1, wherein said third face is aplane.

3. A tube in accordance with claim 1, wherein said third face is aconcave surface.

4. A tube in accordance with claim 1, wherein said third face iscomposed of at least two intersecting surfaces.

5. A tube in accordance with claim 4 wherein said intersecting surfacesare two planes, the one being substantially perpendicular and the othersubstantially parallel to said first face.

6. In an electron tube having a delay line and an elec trode opposedthereto and defining therewith an electron and wave interaction space,means for positively biasing said line with respect to said electrode,thereby establishing in said space an electrostatic field having linesof force transverse thereto, means for establishing in said space atransverse magnetic field having lines of force crossed with said linesof force of said electrostatic field, and means comprising an electronsource for propagating electrons into said space, said electrons beingpropagated normally in one direction in said space: said delay linecomprising a series of periodically spaced ele ments, each elementcomprising first and second faces substantially parallel to the lines offorce of said electrostatic and magnetic fields, said first face facingin the direction opposite said direction of propagation of electrons,and a third face intersecting respectively said first and second facesalong first and second edges of said element, the distance between saidfirst edge and said electrode being greater than the distance betweensaid second edge and said electrode so that electrons impinging on saidelements are absorbed by said third faces hereof.

References Cited in the file of this patent UNITED STATES PATENTS2,414,121 Pierce Jan. 14, 1947 2,687,777 Warnecke et a1 Aug. 31, 19542,702,370 Lerbs Feb. 15, 1955 2,730,678 Dohler et al. Jan. 10, 19562,774,913 Charles Dec. 18, 1956 FOREIGN PATENTS 677,587 Great BritainAug- 20, 1952

